I would assume the box is bigger because fiberglass can be molded to fit the underdash area better. The JL specs call for 0.3 cubic ft enclosure, and the other Smart boxes have just over 0.2 (per others on the forum) and use polyfill to make up the difference in performance. I bet JL squeezes out more volume with this box.
Now...is it worth $200 more for the volume? Not for my pocketbook. If I were all in for every component, maybe, but there are better places I could use the money.
For that much more, you could get a much stronger amp, etc. and people have gotten great results from the existing boxes. Polyfill to make up for volume is an old audio shortcut (as someone in another thread pointed out).